The present invention relates to a transmitting method, a receiving method, a transmitter, and a receiver, and is suitably applicable, for example, to cellular radio communication systems.
In recent years, a cellular radio communication system that is represented by a portable telephone system has been put to practical use and employed in the field of mobile communication. This cellular radio communication system divides an area providing communication services into cells of a desired size, and installs base stations as fixed stations within the cells, respectively. Communication terminal equipment as a mobile station communicates by radio with a base station whose communication state is believed to be most satisfactory.
As for the communication method between the base station and the communication terminal equipment, a wide variety of methods have been proposed. As the representative one, a time-code-division multiple access method combining a time-division multiple access method (TDMA method) and a code-division multiple access method (CDMA method) together has been proposed.
This time-code-division multiple access method, as shown in FIG. 1, divides a band of 1.2 MHz into time cycles of 4.615 ms (this time cycle will hereinafter be called a frame) in a time axis direction. Each frame is further divided at time intervals of 577 xcexcs, thereby dividing each frame into time slots TS0 to TS7. The divided time slots TS0 to TS7 are used as communication channels in a time direction, whereby time-division multiple access is performed.
Also, in this time-code-division multiple access method, as shown in FIG. 2, in each time slot a transmission symbol which is an object of transmission is multiplied by a diffusion code and is transmitted. At this time, eight kinds of diffusion codes are prepared per one time slot, whereby eight communication channels by code division are ensured per one time slot.
Thus, in this time-code-division multiple access method, eight communication channels by code division are ensured per one time slot and also eight time slots are formed in a time direction per one frame of 4.615 ms, whereby 64 communication channels are ensured in 1.2 MHzxc3x974.615 ms.
Now, in the case where transmission is actually performed by this method, a transmission symbol with a symbol rate of 135 ksymbol/s is first diffused into eight times as many spectra by the use of a diffusion code with a speed which is eight times the speed so that the chip rate becomes 1080 kchip/s. The diffused transmission signal is transmitted at the timing of time slots allocated to a local station. This makes it possible to realize the communication based on this time-code-division multiple access method.
Note that in each time slot, as shown in FIG. 3, 28 transmission symbols (224 chips after diffusion) are transmitted at the first half portion of the interval of 577 xcexcs as a whole, then 198 chips of predetermined data are transmitted as a training sequence for the characteristic estimation of a transmission line, and then 28 transmission symbols are transmitted again. Incidentally, after the transmission symbol which is last transmitted, a transmission stop interval called a guard interval is provided. With this guard interval, signal collision by a shift in the signal arrival time is prevented.
Now, for the cellular radio communication system which performs communication by the use of a time-code-division multiple access method such as this, a description will be made with FIG. 4. In FIG. 4, reference numeral 1 denotes a transmitter which is provided in a base station, and 2 denotes a receiver which is provided in communication terminal equipment. In the transmitter 1 of the base station, for example if it is assumed that radio communications of eight channels are present, the transmission symbols S1 to S8 of the eight channels will be respectively input to a CDMA modulation section 1A. The CDMA modulation section 1A has eight kinds of diffusion codes which are in an orthogonal relation with one another, and multiplies the transmission symbol S1 by the first diffusion code of the eight kinds of diffusion codes, thereby performing spectrum diffusion modulation on the transmission symbol S1. Likewise, the CDMA modulation section 1A multiplies the input transmission symbols S2 to S8 by the second to the eighth diffusion codes, respectively, and thereby performs spectrum diffusion modulation on the transmission symbols S2 to S8. Thus, in the CDMA modulation section 1A, the transmission symbols of the channels thus modulated by spectrum diffusion are combined together, and this is output to a time-division transmitting section 1B as a transmission signal S9.
The time-division transmitting section 1B converts the frequency of the input transmission signal S9 to a frequency band allocated to this communication system, and the transmission signal S10 is transmitted through an antenna 1C in a burst manner by the use of the time slot TS0, for example, among the above-mentioned time slots.
On the other hand, in the receiver 2, a reception signal S11 received by an antenna 2A is input to a time-division receiving section 2B. The time-division receiving section 2B fetches the signal component of the time slot TS0 from this reception signal S11 and performs a frequency conversion process on the signal component, whereby a reception signal S12 of the base band corresponding to the transmission signal S9 of the transmitter 1 is obtained and this is output to a matched filter 2C.
The matched filter 2C consists of a shift register, a multiplier, and an adder. For example, if the symbol received by this receiver 2 is taken to be the transmission symbol S1, the same code string as the first diffusion code, used when the transmission symbol S1 is diffused, is used as the coefficient of the multiplier. In the multiplier the chips, output from the taps of the shift register to which the reception signal S12 was input, are multiplied by respective code sequences. The matched filter 2C adds the multiplication results by the adder, thereby restoring the transmission symbol S1.
Thus, if such a transmitter 1 and receiver 2 are provided in the base station and communication terminal equipment respectively, communication by a time-code-division multiple access method can be realized.
Incidentally, when the transmission signal S10 multiplexed with such a time-code-division multiple access method is received and from the received signal the transmission symbol of one channel is restored, if a multiple path, etc., are present in the propagation path, the orthogonal relation of the diffusion codes used in the channels will not be entirely held. Therefore, there is a problem that in a simple matched filter such as the matched filter 2C shown in FIG. 4, the signal components of the other 7 channels interfere and the transmission symbol of the channel cannot be accurately restored.
As a method of avoiding this, a method employing a decoding path called a multi-user detection has been proposed. This method, as shown in FIG. 5, restores all transmission symbols S1 to S8 of eight channels from the reception signal S12 by a multi-user detection 2D, taking mutual interference quantities into consideration. Since the transmission symbol is restored by considering the interference quantity which each channel undergoes, this method can reduce the influence of an interference wave in comparison with the method employing a matched filter and restore the transmission symbol more accurately.
However, in this method employing a multi-user detection, not only channels which must be originally decoded but also all channels multiplexed must be decoded and therefore there is a problem that the process associated with decoding becomes complicated and also the amount of processing increases considerably. Specifically, it has been said that the amount of processing increases by about 82 times in comparison with the method employing a matched filter.
The present invention has been made in view of the aforementioned points, and an object of the invention is to propose a transmitting method, a receiving method, a transmitter, and a receiver which are simple in constitution and can perform satisfactory communication even in an under environment where a multiple path is present.
To solve such problems, in the present invention, a time slot formed by time-division multiplexing is divided into a plurality of blocks in at least a frequency direction. Also, a plurality of subcarriers are formed within the block. One communication channel is formed with a predetermined number of said blocks, thereby forming a plurality of communication channels within the time slot. During communication, information of a transmission object is transmitted with a plurality of subcarriers by the use of a desired communication channel of a plurality of communication channels.
In this way, the time slot formed by time-division multiplexing is divided into a plurality of blocks at least in a frequency direction and also within each block a plurality of subcarriers are formed. One communication channel is formed with a predetermined number of different blocks, whereby a plurality of communication channels is formed within the time slot. With this, one time slot can be multiplexed by communication of multiple carriers separated in frequency. Therefore, with simple constitution, communication can be performed satisfactorily even in an environment where a multiple path is present.